This invention relates generally to the field of antennas and more specifically provides a means of control and optimization of the near field behavior of a microwave transmitting antenna.
Microwave transmitting antennas of the aperture type or equivalent operating at millimeter wavelengths have an equivalent aperture diameter that is many wavelengths that defines a near field region extending as far as hundreds of meters. The near field range (Rnf) of an antenna is defined as a range that is less than Rnf≈D2/λ. This is referred to as the near field boundary. D is the equivalent diameter of the antenna and A is the wavelength, all quantities being in meters. For example, an antenna with a diameter of 1 meter, at a wavelength of 0.003 meters (i.e. 100 GHz), the near field boundary is 333.33 meters. At ranges greater than the near field boundary, i.e. in the far field region, the behavior of the beam formed by the radiation from the antenna is well defined and has an intensity that falls off as the inverse square of the range. Most microwave systems, such as radar and communications, operate over ranges that are exclusively in the far field and near field performance is not a consideration.
There are systems that operate in the near field, such as Active Denial Technology (ADT). In the near field the shape and power density distribution of the radiated beam is complicated and changes considerably as a function of range, aperture shape, focal length, illumination amplitude and phase distribution.
An aperture antenna is one that has an aperture through or from which the electromagnetic fields pass to form a radiate beam or field. Any antenna can be described in terms of an equivalent aperture, thus in general the aperture concept is very broad. To simplify much of the analysis a circular aperture antenna is used to explain the qualitative performance characteristics in a somewhat general manner. However, the shape of the aperture does have an important impact in the near field and will be dealt with as required. Unless otherwise stated, an aperture of diameter D operating at a wavelength λ is used as the basis of analysis. In addition to the shape, wavelength, and diameter, the aperture also has another attribute, focal length, f. The focal length is defined as the radius of curvature of the spherical phase front at the aperture.
For the applications under consideration it is desirable to provide a nearly uniform power density distribution, bounded by a minimum and maximum level, over a target area for a continuous variation of range from a few meters from the antenna to a maximum of tens or hundreds of meters. The near field power density of a circular aperture with uniform illumination has a peak on boresight at a typical normalized range on the order of Rnf/6 to Rnf/4 depending primarily on the focal length and shape of the aperture. The first peak of the power intensity on boresight, as the range is decreased from the near field boundary is called the Fresnel maximum. This characteristic is illustrated in FIG. 1. The radial power intensity of the spot is illustrated in FIG. 2. As the focal length is reduced, the power density peak rises and the range of the peak decreases. At ranges closer than the Fresnel maximum peak the power density on boresight has numerous nulls and the shape of the “spot” develops various patterns of rings. As the range increases beyond the Fresnel maximum the “spot” has a central concentration and gradually transitions into the far field where the power density falls off as the inverse square of the range.
When the focal length is made negative, that is the radius of curvature of the phase front is convex instead of concave, the behavior of the normalized boresight power density behaves as shown in FIG. 3. As expected, the power is dispersed by the convex phase front and, as shown in FIG. 3, the power density becomes lower as the negative focal length becomes more convex. When the focal length is negative, as in FIG. 3, the far field performance is seriously degraded. Thus, one would never use a negative focal length for a far field application.
The complexity of the “spot” power density distribution in the near field is illustrated in FIG. 2. The power density of a circular aperture with an infinite focus is plotted as a function of the radial distance from boresight for normalized ranges (R/Rnf) of 0.05, 0.10, 0.15, 0.20 and 0.25. Because of the circular symmetry, the beam profile is a figure of revolution of the plots shown in FIG. 2. The pattern of the power density in the beam is quite variable as a function of range. In addition, for all ranges the total power of the beam is confined to about the same outer diameter although the distribution is non-uniform.
These characteristics are not ideal for applications that require a concentration of the beam power that is confined to an area and does not vary greatly in magnitude over the concentration area. It is desirable to have control of the spot characteristics. In principle it is computationally possible to program the focal length of the aperture such that a more uniform power density distribution is achieved at selected ranges. This is very difficult to implement in that it would require an aperture phased array of hundreds of thousands of elements or a precisely mechanically deformable aperture. Neither of these options is feasible as a practical matter.
How to accomplish a more uniform power density distribution and control of the spot characteristics in the near field region using a practical approach is the subject of the present invention.